Artificial insemination is an important technology in the field of animal husbandry. Artificial insemination of livestock has so far been carried out mainly with cows. However, the conception rate still needs to be improved, and, in terms of livestock other than cows, the artificial insemination technology has not yet been established.
For example, since natural crossing is mainly carried out in the Japanese swine industry, boars must be kept and a high cost is therefore required. Further, since the body weight of a boar becomes not less than 300 kg, the operation is accompanied by a risk. Further, since much labor is required for natural crossing, efficient development of good breeds (e.g., improvement to obtain races excellent in the meat quality and the litter size) by crossing has been prevented.
At present, in some cases in the Japanese swine industry, artificial insemination is carried out. However, since, in the current system, liquid semen is sent only after ordering by pig farmers, the time lag may results in missing of estrus in cases where the estrus suddenly occurred.
Therefore, development of a technology which allows storage of frozen semen in each pig farmer and thereby enables to deal with sudden occurrence of estrus of a sow has been demanded. However, in the cases of artificial insemination using frozen semen in pigs, the mobility of the sperm is remarkably poor after freeze-thawing; the fertilization ability of the sperm is low; the conception rate varies depending on factors such as the season and the race; the conception rate is low; and the litter size is small (Non-patent Literature 1 and Non-patent Literature 2); so that artificial insemination using frozen semen is not carried out almost at all in the field of the swine industry.